Late-blooming John Isner, 26, is ranked 11th and has beaten No. 1 Novak Djokovic and No. 3 Roger Federer this year. He's a medal threat in singles and is courting Serena Williams as a doubles partner.

American tennis star John Isner has improved his world ranking to No. 11. (Cody Duty / Houston Chronicle…)

John Isner is suddenly leaving a large footprint on the men's professional tennis circuit.

The 26-year-old late bloomer has reached a career-high ranking of No. 9 in the world this year (he's No. 11 this week). He briefly became the top-rated American in the world, defeated No. 1-ranked Novak Djokovic at Indian Wells and No. 3 Roger Federer in Davis Cup play, and has led the underdog U.S. to stunning upsets of Switzerland and France on red clay in away Davis Cup matches.

He has also become an Olympic contender and maybe even a power broker.

Isner, who next plays at an ATP clay court event in Madrid as he begins his preparation for the French Open, said that as much as he hopes to win his first major tournament this year, taking an Olympic medal could be just as important to him.

And he's been lobbying Serena Williams to join him in Olympic mixed doubles. Williams is sort of like the most popular would-be prom date when it comes to mixed doubles at the London Games. As Isner's coach, Craig Boynton, says, "How can you not want to play with her? It's an Olympic medal just begging to be won."

As recently as last year, Isner didn't consider himself a strong contender for any Olympic medal and certainly not a contender in the Serena sweepstakes. It was assumed longtime U.S. No. 1 Andy Roddick and Mardy Fish would be on the short list as well as either Mike or Bob Bryan, who are the world's top-ranked men's doubles team.

But suddenly this season it is Isner who is stamping himself as not only America's best hope for a men's Olympic singles medal in the tournament that will be played on Wimbledon's grass, but also for a major title as well, something that hasn't happened since Roddick won the 2003 U.S. Open.

Isner, who is 6 feet 9, owns a first serve that No. 2-ranked Rafael Nadal has called the one shot in tennis that can control the game, and a penetrating forehand that he has improved markedly. The American has also taken a big gulp of confidence this year with those signature wins over Federer and Djokovic.

"Beating Federer was the precursor," Boynton said. "And then John competed well throughout Indian Wells and beat Novak and played Roger tough in the finals. But I don't think Indian Wells happens without that Davis Cup win over Roger. That was the steppingstone."

Isner has taken an unconventional path to the top 10. He not only played college tennis, he played four years of it at the University of Georgia.

His demeanor on the court — where he often acts as though hitting one more tennis ball might be the most unpleasant thing anyone has ever had to do — made it seem as if he didn't care.

But Isner's perseverance in history's longest tennis match, the first-round Wimbledon battle two years ago against Nicolas Mahut of France that played out over three days and took 11 hours 5 minutes before Isner won, 6-4, 3-6, 6-7 (7), 7-6 (3), 70-68, gave tangible proof of how much he wants to win.

And his play at the end of last season gave an idea of what was in store this year.

He made the semis of a tournament in Paris, beating top-five player David Ferrer and top-20 player Feliciano Lopez along the way before having match points against another top-10 player, Jo-Wilfried Tsonga, before he lost. "That was a breakthrough tournament," Boynton said.

Patrick McEnroe, general manager of player development for the United States Tennis Assn., said he saw an earlier breakthrough in Isner's first-round loss to Nadal at the 2011 French Open. Isner took Nadal, the eventual champion, to five sets in a tournament in which Nadal has lost only one match in the last seven years.

"John realized at that point he's got the kind of game nobody wants to deal with," McEnroe said. "If he was going to have a chance to win majors and be in the top 10, he needed to commit to going after every ball and going for winners every ball he could. That's what he did."

Now McEnroe puts Isner with Andy Murray and Tomas Berdych among the handful of players who might take a major from Djokovic, Federer or Nadal in the near future.

Isner said he never expected to rise like a meteor.

"Big guys take longer to develop," he said. "For me it's just doing some improving every year. I still need to get stronger.

"And then doing the things to tailor my game to each surface. For example, on grass, I need to get lower because the ball doesn't bounce as high, it skids. On grass you want to be aggressive, you don't want to play long, drawn-out points. The attacking player usually wins."

After Isner upset Djokovic at Indian Wells, Djokovic said, "It's frustrating when somebody serves over 70% of the first serves in with that angle and with that speed and accuracy. It's a matter of belief, I think, in the end, and confidence on the court."

It is at the majors where Isner is still looking for a major result. His best finish was a quarterfinal at the 2011 U.S. Open. He went out in the third round of this year's Australian Open to Lopez in five sets.

He has had his eye on the Olympics ever since he watched Andre Agassi win singles gold in Atlanta in 1996.

"I was very young," Isner said, "and I didn't know what my tennis career held. Now I get the opportunity to match his effort. That's cool."